### Home Classes Methods

### In Files

- lib/packetfu/protos/udp.rb

### Parent

PacketFu::Packet

### Methods

- ::can_parse?
- ::new
- #peek_format
- #read
- #udp_calc_sum
- #udp_recalc

### Included Modules

- PacketFu::EthHeaderMixin
- PacketFu::IPHeaderMixin
- PacketFu::UDPHeaderMixin

### Files

- .document
- README.rdoc

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- PacketFu::HSRPHeaderMixin
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# PacketFu::UDPPacket

UDPPacket is used to construct UDP Packets. They contain an EthHeader, an IPHeader, and a UDPHeader.

## Example

    udp_pkt = PacketFu::UDPPacket.new
    udp_pkt.udp_src=rand(0xffff-1024) + 1024
    udp_pkt.udp_dst=53

    udp_pkt.ip_saddr="1.2.3.4"
    udp_pkt.ip_daddr="10.20.30.40"

    udp_pkt.recalc
    udp_pkt.to_f('/tmp/udp.pcap')

## Parameters

    :eth
      A pre-generated EthHeader object.
    :ip
      A pre-generated IPHeader object.
    :flavor
      TODO: Sets the "flavor" of the UDP packet. UDP packets don't tend have a lot of
      flavor, but their underlying ip headers do.
    :config
     A hash of return address details, often the output of Utils.whoami?

### Attributes

eth_header[RW]
ip_header[RW]
udp_header[RW]

### Public Class Methods

can_parse?(str) click to toggle source

    # File lib/packetfu/protos/udp.rb, line 44
    def self.can_parse?(str)
            return false unless str.size >= 28
            return false unless EthPacket.can_parse? str
            return false unless IPPacket.can_parse? str
            return false unless str[23,1] == "\x11"
            return true
    end

new(args={}) click to toggle source

    # File lib/packetfu/protos/udp.rb, line 63
    def initialize(args={})
            @eth_header = EthHeader.new(args).read(args[:eth])
            @ip_header = IPHeader.new(args).read(args[:ip])
            @ip_header.ip_proto=0x11
            @udp_header = UDPHeader.new(args).read(args[:icmp])
            @ip_header.body = @udp_header
            @eth_header.body = @ip_header
            @headers = [@eth_header, @ip_header, @udp_header]
            super
            udp_calc_sum
    end

### Public Instance Methods

peek_format() click to toggle source

Peek provides summary data on packet contents.

    # File lib/packetfu/protos/udp.rb, line 127
    def peek_format
            peek_data = ["U  "]
            peek_data << "%-5d" % self.to_s.size
            peek_data << "%-21s" % "#{self.ip_saddr}:#{self.udp_sport}"
            peek_data << "->"
            peek_data << "%21s" % "#{self.ip_daddr}:#{self.udp_dport}"
            peek_data << "%23s" % "I:"
            peek_data << "%04x" % self.ip_id
            peek_data.join
    end

read(str=nil, args={}) click to toggle source

    # File lib/packetfu/protos/udp.rb, line 52
    def read(str=nil, args={})
            raise "Cannot parse `#{str}'" unless self.class.can_parse?(str)
            @eth_header.read(str)
            if args[:strip]
                    udp_body_len = self.ip_len - self.ip_hlen - 8
                    @udp_header.body.read(@udp_header.body.to_s[0,udp_body_len])
            end
            super(args)
            self
    end

udp_calc_sum() click to toggle source

udp_calc_sum() computes the UDP checksum, and is called upon intialization. It usually should be called just prior to dropping packets to a file or on the wire.

    # File lib/packetfu/protos/udp.rb, line 77
    def udp_calc_sum
            # This is /not/ delegated down to @udp_header since we need info
            # from the IP header, too.
            checksum = (ip_src.to_i >> 16)
            checksum += (ip_src.to_i & 0xffff)
            checksum += (ip_dst.to_i >> 16)
            checksum += (ip_dst.to_i & 0xffff)
            checksum += 0x11
            checksum += udp_len.to_i
            checksum += udp_src.to_i
            checksum += udp_dst.to_i
            checksum += udp_len.to_i
            if udp_len.to_i >= 8
                    # For IP trailers. This isn't very reliable. :/
                    real_udp_payload = payload.to_s[0,(udp_len.to_i-8)]
            else
                    # I'm not going to mess with this right now.
                    real_udp_payload = payload
            end
            chk_payload = (real_udp_payload.size % 2 == 0 ? real_udp_payload : real_udp_payload + "\x00")
            chk_payload.unpack("n*").each {|x| checksum = checksum+x}
            checksum = checksum % 0xffff
            checksum = 0xffff - checksum
            checksum == 0 ? 0xffff : checksum
            @udp_header.udp_sum = checksum
    end

udp_recalc(args=:all) click to toggle source

udp_recalc() recalculates various fields of the UDP packet. Valid arguments are:

    :all
      Recomputes all calculated fields.
    :udp_sum
      Recomputes the UDP checksum.
    :udp_len
      Recomputes the UDP length.

    # File lib/packetfu/protos/udp.rb, line 112
    def udp_recalc(args=:all)
            case args
            when :udp_len
                    @udp_header.udp_recalc
            when :udp_sum
                    udp_calc_sum
            when :all
                    @udp_header.udp_recalc
                    udp_calc_sum
            else
                    raise ArgumentError, "No such field `#{arg}'"
            end
    end

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